Debugging methods:
in mongo:
db.currentOp(true).inprog.forEach(
function(d){
if(d.ns == "test.collection" && d.query.query["_id.day"] )
printjson(d);
})
a terminal-friendly syntax:
$ hadoop jar... ...mongo.input.query='{"_id.day":{"$lt":{"$date":"2014-01-19T23:00:00Z"}}}'